Generally, a gas sensor senses a gas leakage concentration, and informs of the gas leakage through an external display or an alarm, so that an accident due to a gas leakage can be prevented.
The gas sensor is made by using a metal oxide semiconductor by applying a sintering process. Then the gas sensor is heated on the inside or outside thereof to a certain temperature. Then the sensor element of the gas sensor composed of the metal oxide semiconductor is made to contact an object gas, and then, the varying electric conductivity of the sensor element is measured, thereby recognizing the gas leakage concentration.
The sensing principle of the gas sensor is known to be as follows.
1) Owing to an absorption accompanied by a dissociation of gas and an oxidizing reaction, a migration of electrons occurs between the absorbed molecules and the gas sensor, with the result that the electron density is varied. PA1 2) Owing to a gate action, i.e., a gas absorption, a depleted layer is formed on the surface of the thin film or on the surface of the sintered body. PA1 3) Owing to oxidations and reductions, the chemical composition of the gas sensor is varied. PA1 4) Owing to the self heat generation, the temperature rises, with the result that the electric resistivity is varied.
Due to the material properties, the gas sensor is easily contaminated by the environmental factors such as moisture and miscellaneous gases, and therefore, the gas sensor shows malfunctions in the initial operating days. Therefore, in order to stabilize the characteristics of the gas sensor, the gas sensor is subjected to an aging process after its manufacture by heating it by passing an electric current through it for several hours or several days.
The above described sensor is disclosed in Japanese Patent Application Laid-open No. Hei-5-209855. In this sensor, in order to dissipate a dew formation and to sense a steep humidity variation, a heat generating body is installed on the sensor body to heat the sensor.
Further, in this conventional gas sensor, in order to measure a stabilized resistivity at the manufacturing step, it has to be waited for several days in a state with an electric current flowing, and therefore, the productivity is lowered. Further, a lengthy time period is required before the sensor is restored to the original state after once sensed a gas leakage.